1. Field of the Invention
This invention relates to a method of casting a metal or ceramic powder which comprises suspending the powder into a liquid dispersion medium to form a slurry and casting the slurry into a mold.
2. Description of the Prior Art
A casting method is known as a molding method for metal powder, ceramic powder, and a mixture of metal powder and ceramic powder.
The inventors disclosed in Japanese Patent KOKAI No. 62-192502 a molding method for metal powder or ceramic powder which comprises suspending the powder in a dispersion medium of which the principal component is a material having a melting point of 0.degree. to 100.degree. C. being extractable with liquid or supercritical carbon dioxide, and casting the slurry into a mold which does not absorb liquid. In this method, the cast slurry is cooled to freeze it, and then demolded. Subsequently, the principal component of the dispersion medium in the frozen body is extracted with liquid or supercritical carbon dioxide. The extracted body is heated, and the residual dispersion medium is removed by thermal decomposition. The molded body thus obtained is densified by sintering to obtain a sintered body. An outline of the above process is shown in FIG. 8. The sintered body is machined if necessary, and used for a cutting tool, a machine part or the like.
The above casting method of metal or ceramic powder is excellent in obtaining a molded body in a short time without the generation of cracks, but has the following problems. In the above method, a volume change of the slurry occurs during freezing. For example, when paraffin wax is used as the dispersion medium, a volumetric shrinkage of about 25% occurs. The freezing begins at the portion touching the mold, and strain is generated on the inside of the treated body by the freezing of the outside. Therefore, the frozen body is more or less deformed. By the deformation, the molded body is sometimes broken during demolding. The degree of the deformation sharply varies according to the temperature of the slurry, casting pressure, the temperature of the mold cooling dispersion medium and pressing time, and therefore, suitable casting conditions must be set requiring trial and error experiments for a long time. The internal strain which occurred in the molding process is released during the extraction with a supercritical fluid or a liquefied gas, thermal decomposition or sintering, and deformation proceeds to decrease size accuracy. In the case of a large profile body, the above problems are more remarkable.